Control units are used in a large variety of processes in process metrology in order to control process variables. Controls in processes with considerable, and especially variable downtimes face particular requirements.
Such a situation may occur, for example, in the field of wastewater treatment and/or water purification. The processes conducted in wastewater treatment plants to treat liquids, e.g. water containing sludge, are usually characterized by slow flow velocities of the liquid to be treated.
In the following, this is illustrated in more detail on the basis of several specific processes in the field of wastewater treatment. The basic problem definition as well as the solution according to the invention described below are, however, transferable to the control of process variables in other processes with downtime characteristics, especially to processes to treat liquids.
The purification of wastewater in wastewater treatment plants usually includes the elimination of nitrogen, organic compounds and phosphate. For this reason, activated sludge processes, also referred to as aeration processes, are employed that contain a biological wastewater purification process. In these processes, the substances to be degraded are converted by microorganisms, and the content of such substances in the wastewater is thus reduced. Activated sludge processes allow the conversion and/or degradation of organic substances and nitrogen compounds. For example, by means of an activated sludge process, the chemical oxygen demand (COD), the biochemical oxygen demand (BOD) and nitrogen compounds (ammoniacal nitrogen content) may be lowered. One measure for the content of organic compounds in a liquid is also the SAC parameter (spectral absorption coefficient), which states the absorption and/or extinction of radiation of one or several wavelengths, especially the extinction of radiation with the wavelength 254 nm in relation to 1 m of the penetrated liquid. It is possible to use one or several further wavelengths as reference. A reduction of organic compounds with an activated sludge process thus consequently leads to a reduction of the SAC.
In an activated sludge process, the traditional degradation of nitrogen occurs, for example, by means of the processes of nitrification and denitrification. The term nitrification refers to the bacterial oxidation of ammonium to nitrate. Denitrification is understood to be the conversion of the nitrogen contained in the nitrate to molecular nitrogen by bacteria called denitrificants. While the nitrification process requires an oxygen supply for oxidation of the nitrogen compounds, the denitrification process has to be conducted under anoxic conditions, wherein the denitrificants must have sufficient supply of oxidizable substances available. Two different types of methods have primarily been established, which both allow the realization of these different conditions for the two processes, namely continuous methods and intermittent methods. In a continuous method, the denitrification is conducted in a first tank, the denitrification tank, and the nitrification in a second tank, the nitrification tank, wherein the two tanks may also be further subdivided. The denitrification tank is operated anoxically while the nitrification tank is supplied with air through an aerator. The liquid in the nitrification tank is partly pumped back into the denitrification tank (“internal recirculation”) to continue the conversion of nitrate formed during the nitrification process into nitrogen. The remaining part of the nitrification tank effluent flows on to secondary treatment. In intermittent methods, nitrification and denitrification are completed in a single tank, with the tank intermittently being aired by means of an aerator that is turned on and off.
The first automated wastewater treatment plants worked purely time-controlled with intermittent methods. For continuous methods, the aeration in the nitrification tank was operated permanently or was controlled via a timer. Later on, controls and means of controlling the aeration became known that were based on the measurement of the concentration of dissolved oxygen or the redox potential of the liquid to be treated. More recently, online-capable sensors for the ammonium and nitrate measurements have become known, so that now an observation of the ammonium and nitrate concentration in real-time and thus also controls based on such ammonium and nitrate measurements have become possible.
Since in a continuous method the liquid to be treated flows through the treatment tanks, it follows that a measuring point used to determine a control variable is arranged downstream of the aeration in flow direction. For an intermittent method, too, measuring points for dissolved oxygen or ammonium usually are arranged inside the tank in the area of the fluid discharge or even directly in the tank drain. This has the disadvantage that load variations of the nitrogen compounds to be degraded are only captured, i.e. “noticed”, by the control unit when they have arrived at the measuring point. Any load fall thus leads to too much air being supplied into the tank. The associated energy consumption is thus higher than necessary. If a load rise is noted only in the area of liquid discharge, aeration cannot be activated quickly enough to provide sufficient oxygen for the nitrification process in order to degrade the entire nitrogen or organic load. In such cases, there is a danger that outlet threshold values are exceeded. In adverse load variation cases, it may also happen that the controls start to build up and/or swing.
A regulation or control of the aeration based on load prognoses that take into account expected load variations e.g. on the basis of known diurnal variation is conceivable, but cannot be realized easily. One of the reasons for this is that the nitrogen load to be degraded is influenced by a variety of effects that are difficult to forecast, such as, for example, weather influences or irregular discharge of industrial wastewater in the catchment area of the wastewater treatment plant.
There are further considerations of capturing measuring values in the intake area of the wastewater treatment plant, before the aeration tank and/or nitrification tank and use them as regulating variables for the aeration. Any modification of the measuring values that are considered for the determination of the regulating variable for the aeration, however, is only included at the place of the aerator with a certain delay that depends on the flow velocity of the liquid to be treated. It is therefore not possible with the methods known from the current state of the art to adjust the aerator very precisely to provide aeration that is adjusted as exactly as possible to the load of substances to be degraded found in the liquid that is in the aeration tank near the aeration area at that moment.